Electrochemical gas sensors are applied in terms of exhaust gas oxygen sensors (lambda sensors) in exhaust gas systems of combustion engines in motor vehicles, in order to provide signals over the exhaust gas composition for the motor control. Hereby the engine can be operated so, that the exhaust gases show an optimal composition for the post-treatment with the catalysts, that are usually present in an exhaust gas system nowadays.
FIG. 1 displays a gas sensor, which is known from the state of the art and pursuant to this category. The sensor element 100 presents a gas entry leak 115, through which exhaust gas flows in and arrives at a measurement chamber 130 through a diffusion barrier 120. An inner pump electrode 140 is arranged in the measurement chamber. An outer pump electrode 150, which is arranged at the outside of the solid electrolyte 110 and under a porous protective layer 155, is exposed to the exhaust gas of a (not further shown) combustion engine.
A pump voltage Upump is applied between the inner pump electrode 140 and the outer pump electrode 150, so that a pump current Ipump flows. A heating device 160, which is embedded in an isolation layer 162, is furthermore arranged in the solid electrolyte 110. Via the heating device 160 the sensor element 100 is warmed up to a temperature, which allows an optimal functioning of the sensor element 100.
This planar broad-band lambda sensor according to the principle of limited current is admitted with a solid pump voltage Upump. At a lean exhaust gas, meaning an exhaust gas with air surplus, the solid pump voltage Upump produces a positive pump current Ipump, which is clearly connected with the oxygen content of the exhaust gas. In a rich exhaust gas, meaning an exhaust gas with fuel surplus, there is a positive pump current as well, but due to the decomposing of the water contained in the exhaust gas.
The applied pump voltage Upump is indeed lying clearly under the decomposition voltage of the water, but since hydrogen exists in the exhaust gas, the water decomposition is energetically possible, because of the production of water from the reaction between hydrogen and oxygen at the outer pump electrode 150. The pump current Ipump is thus limited at a rich exhaust gas by the hydrogen content. Since this pump current Ipump shows the same direction in rich exhaust gas like the pump current Ipump in a lean exhaust gas, the exhaust gas composition cannot be implied anymore from the pump current Ipump.
The task of the present invention is to improve a gas sensor pursuant to this category, so that the exhaust gas composition can be implied from a lean as well as from a rich exhaust gas.